DAC8574EVM

1.1 Features

This EVM features the DAC8574 digital-to-analog converter (DAC). The DAC8574 EVM is a simple evaluation module designed for a quick and easy way to evaluate the functionality and performance of the high resolution, quad-channel, and serial I2C input DAC. This EVM features a serial I2C interface to communicate with any host microprocessor or TI DSP base system.

1.2 Power Requirements

The following sections describe the power requirements of this EVM.

1.2.1 Supply Voltage

The dc power supply requirement for the digital section (VDD) of this EVM is typically 5 V connected to the J5-1 or via J6-10 terminal (when plugged in with another EVM board or interface card) and is referenced to ground through the J5-2 and J6-5 terminal. The dc power supply requirements for the analog section of this EVM are as follows; the VCC and VSS ranges from 15.75 V to-15.75 V maximum and connects through J1-3 and J1-1 respectively, or through J6-1 and J6-2 terminals. The 5 VA connects through J5-3 or J6-3 and the 3.3 VA connects through J6-8. All of the analog power supplies are referenced to analog ground through J1-2 and J6-6 terminals.

The device under test, U1, analog power supply can be powered by either 5 VA or 3.3 VA by selecting the proper position of jumper W1. This allows the DAC8574 analog section to operate in either supply power while the I/O and digital section is powered by 5 V, VDD.

The VCC supply source is mainly used to provide the positive rail of the external output op-amp, U2, the reference chip, U3 and the reference buffer, U8. The negative rail of the output op-amp, U2, can be selected between VSS and AGND via W5 jumper. The external op-amp is installed as an option to provide output signal conditioning or boost capacitive load drive and for other output mode requirement desired.

1.2.2 Reference Voltage

The 5-V precision voltage reference is provided to supply the external voltage reference for the DAC through REF02, U3, via jumper W4 by shorting pins 1 and 2. The reference voltage goes through an adjustable 100 kΩ potentiometer, R11, in series with 20 kΩ, R10, to allow the user to adjust the reference voltage to its desired settings. The voltage reference is then buffered through U8A as seen by the device under test. The test points TP1, TP2 and TP5 are also provided, as well as J4-18 and J4-20, to allow the user to connect other external reference source if the onboard reference circuit is not desired. The external voltage reference should not exceed 5-V dc.

The REF02 precision reference is powered by VCC (15 V) through J1-3 or J6-1 terminal.

1.3 EVM Basic Functions

The DAC8574 EVM is designed primarily as a functional evaluation platform to test certain functional characteristics of the DAC8574 DAC. Functional evaluation of the DAC device can be accomplished with the use of any microprocessor, TI DSP, or some sort of a waveform generator.

The headers J2 and P2 are the connectors provided to allow the control signals and data required to interface a host processor or waveform generator to the DAC8574 EVM using a custom built cable.

A specific adapter interface card is also available for most of TI’s DSP starter kit (DSK) and the card model depend on the type of the TI DSP starter kit to be used. Make sure to specify the DSP that is used to interface with to acquire the right adapter interface card. In addition, there is also an MSP430 based platform (HPA449) that uses the MSP430F449 microprocessor, which this EVM can connect to and interface with. For more details or information regarding the adapter interface card or the HPA449 platform, please call Texas Instruments Inc.

The DAC outputs can be monitored through the selected pins of J4 header connector. All the outputs can be switched through their respective jumpers W2, W11, W12 and W13, for the reason of stacking. Stacking allows a total of eight DAC channels to be used provided that the I2C address is unique for each EVM board stacked.

In addition, the option of selecting one DAC output that can be fed to the noninverting side of the output op-amp, U2, is also possible by using a jumper across the selected pins of J4. The output op-amp, U2, must first be configured correctly for the desired waveform characteristic, refer to Section 3 of this user’s guide.

Figure 1-1. EVM Block Diagram

2.1 PCB Layout

The DAC8574 EVM is designed to preserve the performance quality of the DAC, device under test, as specified in the data sheet. Carefully analyzing the EVM’s physical restrictions and the given or known elements that contributes to the EVM’s performance degradation is the key to a successful design implementation. These obvious attributes that diminish the performance of the EVM can be taken care of during the schematic design phase, by properly selecting the right components and building the circuit correctly. The circuit must include adequate bypassing, identifying and managing the analog, and digital signals and knowing or understanding the components mechanical attributes.

The obscure part of the design is the layout process where lack of knowledge and inexperience can easily present a problem. The main concern here is primarily with the placement of components and the proper routing of signals. The bypass capacitors should be placed as close as possible to the pins and the analog and digital signals should be properly separated from each other. The power and ground plane is very important and must be carefully considered in the layout process. A solid plane is ideally preferred but sometimes impractical, so when solid planes are not possible, a split plane can do the job as well. When considering a split plane design, analyze the component placement and carefully split the board into its analog and digital sections starting from the device under test. The ground plane plays an important role in controlling the noise and other effects that otherwise contributes to the error of the DAC output. To ensure that the return currents are handled properly, route the appropriate signals only in their respective sections, meaning the analog traces should only lay directly above or below the analog section and the digital traces in the digital section. Minimize the length of the traces, but use the biggest possible trace width allowable in the design. These design practices discussed can be seen in the following figures presented on the following pages.

The DAC8574 EVM board is constructed on a four-layer printed-circuit board using a copper-clad FR-4 laminate material. The printed-circuit board has a dimension of 43,1800 mm (1.7000 inch) X 82,5500 mm (3.2500 inch), and the board thickness is 1,5748 mm (0.0620 inch). Figures 2-1 through 2-6 show the individual artwork layers.